Hybridization in the Cetacea: widespread occurrence and associated morphological, behavioral, and ecological factors.

Hybridization has been documented in a many different pairs of cetacean species both in captivity and in the wild. The widespread occurrence of hybridization indicates that postmating barriers to interbreeding are incomplete within the order Cetacea, and therefore raises questions about how species integrity is maintained in the face of interspecific (and often intergeneric) gene flow. We examined hybridization across the order Cetacea (oceanic species included: N = 78; species with 44 chromosomes included: N = 52) to test for associations between the occurrence of hybridization and similarity across 13 ecological, morphological and behavioral traits in hybridizing vs. non-hybridizing species pairs. We found that species pairs that share a greater number of traits had a higher propensity to hybridize than pairs of species that did not. This trend was driven by behavioral and morphological traits such as vocalization frequency and body size. Together our findings suggest the importance of divergent selection on morphological and behavioral traits within sympatric species in constraining opportunities for hybridization and preventing the collapse of parental species.